Generally, an electronically controlled automatic transmission achieves the automatic shift operation in accordance with a fixed shift pattern as shown in FIG. 1.
That is, the automatic transmission determines each speed ratio in accordance with a fixed shift pattern programmed in a computer (i.e., a transmission control unit ("TCU")), while the manual transmission performs the shift operation according to the driver's will.
Therefore, since the shift timing is determined by the fixed shift pattern, the engine brake effect cannot be obtained.
For example, in the shift pattern of the convention al automatic transmission, as shown in FIG. 1, the X-axis represents the number of rotation of the transfer gear of vehicle speed and the Y-axis represents a throttle opening relating to an amount the accelerator is depressed by the driver.
The automatic transmission is used on many rear-wheel-drive and four-wheel-drive vehicles. Automatic upshifts and downshifts are a convenience for the driver because a foot-operated clutch is not required to shift gears and because the vehicle can be brought to a stop without the use of a clutch and without shifting the transmission into neutral.
An automatic transmission as illustrated in FIG. 2 comprises a torque converter, an A/T 22 having a TCU and a hydraulic control system, and a planetary gear unit.
As shown in FIG. 3, the torque converter transmits driving force of the engine to a transmission and has a function as a flywheel of a manual transmission. In addition, the torque converter boosts engine torque when starting and accelerating the vehicle and absorbs the torque change, thereby providing improved ride comfort.
The torque converter is filled with the fluid therein, and by using centrifugal force of the fluid, transmits rotating force of the engine to wheels while absorbing the impact and distortion of the crankshaft.
The pump or impeller 32 converts the rotating force of the engine into the centrifugal force, and transmits the centrifugal force to the turbine 31, which transmits the centrifugal force of the fluid transmitted from the pump 32 to an input shaft 35. A stator 33 converts a direction of the fluid transmitted from the turbine 31 and the transmitted fluid is used to increase the torque.
In recent years, a new control method, for example, a fuzzy control or a neutral control method, has been used for controlling the automatic transmission vehicle so as to perform the shift operation.
Although, this kind of control algorithm is very effective through a control of the shift timing under various road driving states, when an emergency circumstance occurs, that is an engine brake is required during driving a vehicle, since all the conventional control method controls the damper clutch to be released, the engine brake effect deteriorates.
In the conventional damper clutch control method is for improving the fuel consumption ratio. That is, by controlling the pump and turbine to be integrated to each other, the damper clutch is engaged in a predetermined range obtained through a test as shown in FIG. 4.